Summary of heat treatment knowledge

Heat treatment refers to a kind of metal hot working process in which materials are heated, insulated and cooled in solid state to obtain the expected structure and properties. Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the performance of the workpiece by changing the microstructure inside the workpiece or the chemical composition on the surface of the workpiece. The characteristic of heat treatment is to improve the internal quality of workpiece.

Heat treatment process generally includes three processes: heating, heat preservation and cooling, sometimes only heating and cooling. These processes are interlinked and uninterrupted. 

Heating is one of the important processes of heat treatment.

As for the direct cracking of heat treated steel after quenching, the following reasons can be referred to:

1. The wrong heat treatment process is not suitable.

2. Improper cooling, rapid cooling below MS temperature, excessive stress.

3. The difference of cross-section size of workpiece is too large, or there are many holes, or there are places with stress concentration.

4. The original structure is poor, the raw material has network eutectic carbide or poor spheroidizing annealing, or the raw material has micro cracks, and the cracks expand and crack during quenching.

5. The workpiece is not preheated, and the heating speed is too high to cause heating cracking.

6. The quenching temperature is too high, the grains are coarse and brittle. Grain boundary oxidizes or melts during overburning.

7. No intermediate annealing was carried out before repeated quenching.

8. No tempering in time after quenching.

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